1. Field of the Invention
The present invention relates to an image forming apparatus adapted to irradiate light on a surface of an image carrier for detection of light therefrom and to determine a density of an image on the image carrier based on the detection results, and to an image density calculation method therefore.
2. Description of the Related Art
Image forming apparatuses such as copy machines, printers and facsimile machines utilizing the electrophotographic techniques may encounter density variations of toner images associated with individually different characters of apparatuses, variations with time, or changes in conditions surrounding the apparatus which include temperature, moisture and the like. Heretofore, there have been proposed a variety of techniques for ensuring stable image density, which include, for example, a technique wherein a small test patch (patch image) is formed on the image carrier such that a density control factor affecting the image density may be optimized based on the density of the patch image. According to the technique, a desired image density is attained by forming a predetermined toner image on the image carrier with the density control factor set to a different value each time; sensing the density of the toner image, as the patch image, on the image carrier; and controlling the density control factor in a manner to establish coincidence between the density of the patch image and a predetermined target density.
While there have heretofore been proposed a variety of techniques for taking measurement of the patch image density (hereinafter, referred to as “patch sensing technique”), the techniques utilizing optical means are most commonly used. Specifically, the technique takes a procedure including the steps of irradiating light on a surface region of the image carrier or an intermediate member with the patch image formed thereon; detecting light reflected from or transmitted through the surface region by means of an optical sensor; and determining the density of the patch image based on the amount of detected light.
In order to set the density control factor to an optimum value for forming the toner image of good quality, how accurately the density of the formed patch image is sensed is an important point for the image forming apparatus adapted to adjust the density control factor based on the density of the patch image. Unfortunately, the above conventional patch sensing techniques have the following problems.
There may be a case where because of changes in the surface conditions of the image carrier due to mars or dirt thereon or of noise factors such as electrical noises, noises are superimposed on a sensor output so that a sensed value does not indicate a correct density of the patch image. The conventional patch sensing technique for image forming apparatus does not give adequate consideration to such influences of noises. Since the density control factor is adjusted based on the density of the patch image determined from such a sensor output containing the noises, the density control factor may not always be optimized, resulting in a deteriorated image quality.
The incidence and magnitude of noises vary from condition to condition. Therefore, it is quite difficult to identify the nature of noises contained in the sensor output. On this account, a demand exists for the establishment of a technique for achieving high-accuracy determination of the patch image density through effective removal of the noises.